Do people read the verse by Randolph Ash and Christabel LaMotte that AS Byatt has supplied with her novel? Many proudly admit not, even while saying they enjoy Possession, as if the commonsensical reader knows better than to pause over this mock-Victorian poetry. Byatt's American publishers initially wanted her to cut large parts of it, fearing purchasers would be put off.

Certainly the novelist has taken an odd sort of gamble with her pastiches. Most readers will surely not be able to recognise the genres she imitates, the verse forms she mimics, the habits of diction and imagery that she follows. Yet those who do appreciate these will see that the "Browning" blank verse credited to Ash is ploddingly regular stuff compared to the original; that in LaMotte's creations Christina Rossetti's fable-mongering is awkwardly blended with Emily Dickinson's staccato stanzas.

The gamble seems the greater as the poetry has no obvious narrative function, except to serve as a kind of authentication device, hints at a larger imagined world. Formally, indeed, it is not part of the narrative. It is given without explanation. Other elements of pastiche - bits of the "diary" of Ellen Ash, or of the academic biography of Ash written by Mortimer Cropper - serve the plot. The enjoyable parodies of feminist or post-structuralist literary criticism, meanwhile, have sharply satirical purposes.

Yet "quotation" does set the mood for what follows. So, for instance, LaMotte's sub-Dickinson lyric about the violence pent up "behind the blinds" of domesticity heads the chapter where Maud and Roland visit the house LaMotte once shared with her jealous "companion", Blanche.

The poems provide clues to the relationship between Ash and LaMotte - read rightly, the imagery of Melusina shows the poets were in Yorkshire together - but also trigger mistaken interpretations. Byatt relishes showing how, before Maud and Roland get digging, academics have found all the wrong biographical suggestions in the poetry.

"Pastiche" originally meant a medley of different styles, and Byatt has fabricated a variety of texts from which the past is to be pieced together. The pastiche poetry, however, suggests the gap between modern "explanation" and the still mysterious voices of the literary past.

The custom of opening sections of a novel with poetry has a peculiar history. It was pioneered by Ann Radcliffe, in so many ways both a formulaic and an extraordinarily influential novelist. In her fiction, every chapter begins with its epigraph, usually a passage of verse. She especially favoured Shakespeare (notably Macbeth) and moody 18th-century graveyard verse, but she did also compose her own poetry to make for the right atmosphere. Other gothic novelists followed her lead.

Later, Sir Walter Scott took to fabricating appropriate, antique-sounding fragments of verse, meaningfully placed at the heads of chapters. "I believe that, in some cases, where actual names are affixed to the supposed quotations, it would be to little purpose to seek them in the works of the authors referred to," Scott disarmingly admitted at the beginning of Chronicles of the Canongate.

A few Victorian novelists pursued the trend, most teasingly George Eliot, a favourite of Byatt. Middlemarch is full of mock-fragments of verse and drama, carefully forged by Eliot (and invariably passed over without comment by annotators and critics). She clearly delighted in manufacturing these perplexing or gnomic shards of literature, running through various styles and periods, at once "discovering" wisdom and mocking sententiousness. She is surely one model for the invention of old verse in Possession.

For pastiche means mimicry that we enjoy without being fooled. Byatt's versions of Browning are more satisfying in this way than some of LaMotte's verse, where the poetry sometimes seems in earnest. Browning was a ventriloquist, a great writer of dramatic monologues and verse in many different personae. Ash is allowed to be the same, so pastiche playfully reflects the original.

Byatt's pastiches are emphatically not wonderful poetry, yet display considerable technical skill (how many academic critics could produce such things?) and function as a kind of homage to the poetry she admires. They work best while they remain merely amusing copies of the surface qualities of 19th-century verse, flattering the attentive reader. It is only when her imitations get serious that we should worry.

· John Mullan is senior lecturer in English at University College London

· Have your say about Possession on the Guardian talkboards or write to The Review, The Guardian, 119 Farringdon Road, London EC1R 3ER